For the sentence "That is not a menu", why is the translation given as "Das ist kein Menu"? Why can't it be "ein nicht Menu"?
The best you'd be able to do, using "nicht", would be "Das ist nicht ein Menü". This sentence still feels a little clumsy to me compared with "Das ist kein Menü" but I think it's nonetheless valid. Someone more fluent might be able say whether there's a real difference there. Regardless, "ein nicht Menü" just doesn't work.
Nicht, I believe, is usually used to negate a verb, such as when you want to say that you can't do something. It means not. As in "Ich kann es nicht tun". Kein means no or not any. So if, for instance, if you don't have a car you could say "Ich habe kein Auto". Kein ( no or not any) with nouns